[llvm] [NFC][SROA] Clean up rewritePartition type selection process (PR #169106)

[Yonah Goldberg via llvm-commits]

YonahGoldberg wrote:

> I think it might have been cleaner to keep the checkVectorTypeForPromotion() function out of line even if it's used only in one place, but no strong opinion...

Personally, I found the call depth of `isVectorPromotionViable` a little confusing while trying to understand the code for the first time. `isVectorPromotionViable` calls `createAndCheckVectorTypesForPromotion` which calls `checkVectorTypesForPromotion` which calls `checkVectorTypeForPromotion` which calls `isVectorPromotionViableForSlice`. I preferred to flatten it out a bit since it was only used in that one place.

https://github.com/llvm/llvm-project/pull/169106